Conventional laser systems are typically employed for processing target structures such as electrically conductive links in integrated circuits or memory devices such as EPROMs, DRAMs, and SRAMs. Link processing, which is presented herein only by way of example of selective material processing, may include total or partial removal, cutting, or vaporization of the link material. Examples of link processing laser systems include models in the 8000 and 9000 series manufactured by Electro Scientific Industries, Inc., which is the assignee of the present application. These systems typically utilize laser output wavelengths of 1.064 .mu.m, 1.047 .mu.m, and 0.532 .mu.m.
The physics and computer modeling for laser-based link processing are described by L. M. Scarfone and J. D. Chlipala, "Computer Simulation of Target Link Explosion in Programmable Redundancy for Silicon Memory," Journal of Materials Research, vol. 1, No. 2, Mar.-Apr. 1986, at 368-81, and J. D. Chlipala, L. M. Scarfone, and Chih-Yuan Lu, "Computer-Simulated Explosion of Poly-Silicide Links in Laser-Programmable Redundancy for VLSI Memory Repair," IEEE Transactions on Electron Devices, Vol. 36, No. 6, June 1989, at 1056-61.
FIGS. 1A and 1B are respective cross-sectional side elevation and top views depicting a conventional output energy distribution of a laser output or pulse 10 directed at an integrated circuit or memory link structure 12 positioned between link terminators 14 and covered by a protective layer 16 that is often the result of oxide passivation. Link structure 12 may be composed of one or more layers of a single material or a composite "sandwich" of several materials including those required for anti-reflective coating, binding, or other manufacturing purposes. For example, link structure 12 may include sublayers of titanium and tungsten to enhance adhesion between an aluminum base link material and a substrate 22, such as silicon, gallium arsenide, or other semiconductor materials. Substrate 22 may also include oxide layers.
With reference to FIGS. 1A and 1B, Chlipala et al. suggest that a laser pulse 10 focused to a spot 18 of radius R (which is, for example, about 2 .mu.m) and applied across link structure 12 should have a suitable duration or pulse width and be of sufficient energy at a certain wavelength to cause a temperature distribution capable of cutting link structure 12. Because the spatial or critical dimensions of spot 18 are typically (but not always) larger than the width (which is, for example, about 1 .mu.m) of link structure 12, a portion of laser pulse 10 impinges on substrate 22. Laser pulse 10 must, therefore, be tailored not to have energy sufficient to damage substrate 22 or adjacent circuit structures 20 either by direct laser energy absorption, by residual pulse energy coupled into substrate 22 below link structure 12 after it is vaporized, or by thermal conduction.
Yields of memory devices have been dramatically improved by employing 1.064 .mu.m or 1.047 .mu.m laser output to ablate polysilicon, polycide, disilicide, and other similar material link structures 12 to enable redundant memory cells. Even though it is a relatively poor electrical conductor, polysilicon-based material is well-characterized and is easily fabricated to form link structures 12 that can be processed by laser systems at energy levels that do not prohibitively damage substrate 22 when conventional laser wavelengths 1.047 .mu.m or 1.064 .mu.m are used. A laser output having well-controlled energy and power levels can generate a desired temperature distribution within a polysilicon link structure 12 that exceeds the melting temperature of polysilicon. Scarfone et al. attribute this advantage to the relatively large optical absorption depth of polysilicon at 1.064 .mu.m in combination with other favorable parameters such as the mechanical strength, the thermal conductivity, and the melting and vaporization temperatures of polysilicon, protective layer 16, substrate 22, and other materials involved.
The technology trend is, however, toward developing more complex, higher density memories having more layers and smaller link structures 12 and memory cell dimensions. As the complexity and numbers of layers of memory devices have increased, the polysilicon-like links have typically become smaller and more deeply buried and thus more difficult to process at the conventional laser outputs of 1.047 .mu.m or 1.064 .mu.m. Accordingly, an expensive and time-consuming process is typically employed to delicately etch away surface layers to expose the polysilicon-like structures to be processed. Another disadvantage of polysilicon-like materials, including polycide and disilicide, is that their electrical resistance increases with smaller dimensions to values that will restrict the operating speed of the memory cells.
To address the higher electrical resistance and associated signal propagation delay contributed to polysilicon, memory manufacturers have developed and adopted multilayer and multimaterial constructions, including new link structure materials such as aluminum, titanium, nickel, copper, tungsten, platinum, gold, metal nitrides, or other electrically conductive metal-like materials. In general, these new types of link structures and materials are more difficult to process using a 1.047 .mu.m or 1.064 .mu.m wavelength laser because of factors such as higher or differing material melting points and/or differing thermal transition characteristics between different material layers of link structure 12.
These link structures 12 and new materials have a reduced laser power processing window because they generally require a higher power level to cleanly remove the link structure material. Thus, the lower power threshold for effective link processing is effectively raised while the upper power limit remains constant because the substrate damage threshold is unchanged.
Various types of spot-shaping (e.g., uniform, square, oval) were attempted to increase the coupling of energy into the link structure material while minimizing the energy coupled into substrate 22. These attempts achieved limited success and often caused more undesirable side effects.
Simply increasing the laser output power level has deleterious effects on silicon, gallium arsenide, and other semiconductor substrates 22 and adjacent circuit structures 20. On the other hand, increasing the laser output pulse width to provide more time for thermal conduction to redistribute the heat to the underlying volume of a high-conductivity link structure 12 increases the heat conducted into the substrate and the cumulative laser energy of the output pulse, thereby also increasing the risk of damage to substrate 22 and circuit structure 20. Thus, some practitioners have concluded that laser systems are no longer the proper tool for processing high-conductivity links and have discussed using ion beams instead as described by Jon Orloff, "Focused Ion Beams, Scientific American, October 1991, at. 96-101. However, ion beam technology is still largely experimental for such applications, is very expensive, is not an automated production process (i.e., it is very slow), and cannot easily be retrofitted into existing laser-based link cutting systems.